Ultra-High Energy Cosmic Rays and the Connection to -ray and Neutrino Astrophysics accelerated protons interact: => energy fluences in -rays and neutrinos are comparable due to isospin symmetry. The neutrino spectrum is unmodified, whereas -rays pile up below the pair production threshold on the CMB at a few 1014 e. V. The Universe acts as a calorimeter for the total injected electromagnetic energy above the pair threshold. This constrains the neutrino fluxes.

A possible acceleration site associated with shocks in hot spots of active galaxies

Conclusions 1. ) Pion-production establishes a very important link between the physics of high energy cosmic rays on the one hand, and -ray and neutrino astrophysics on the other hand. All three of these fields should be considered together. 2. ) There are many potential high energy neutrino sources including speculative ones. But the only guaranteed ones are due to pion production of primary cosmic rays known to exist: Galactic neutrinos from hadronic interactions up to ~1016 e. V and “cosmogenic” neutrinos around 1019 e. V from photopion production. Flux uncertainties stem from uncertainties in cosmic ray source distribution and evolution. 3. ) The highest fluxes above 1019 e. V are predicted by top-down models, the Z-burst, and cosmic ray sources with power increasing with redshift. 4. ) The coming 3 -5 years promise an about 100 -fold increase of ultra-high energy cosmic ray data due to experiments that are either under construction or in the proposal stage. This will constrain primary cosmic ray flux models. 5. ) Many new interesting ideas on a modest cost scale for ultra-high energy neutrino detection are currently under discussion, see experimental talks.